Gavin Newsom and Ron DeSantis have spent years hurling political and policy grenades from their blue and red state capitols — the personification of a nation riven by partisan warring. But the California Democrat and the Florida Republican now find themselves converging on an issue that’s gained a surprising amount of bipartisan support: outright banning, or severely curtailing, children from using smartphones at schools. That doesn’t mean the popular policy will erase the long-running...